Investigation of Temperature Effect on the Porosity of a Fabric Based Porous Capacitive Pressure Sensor

A fabric based porous polydimethylsiloxane (PDMS) pressure sensor was developed and the effect of curing temperature on the porosity as well as the sensitivity was investigated. Three different porous PDMS dielectric layers (D1, D2 and D3) were prepared by curing a mixture of PDMS, sodium hydrogen bicarbonate (NaHCO
3
), and nitric acid (HNO
3
) at 110 $^{\circ}C$, 140$^{\circ}C$ and 170$^{\circ}C$, respectively. The top and bottom electrodes of the pressure sensor were fabricated by screen printing silver (Ag) on a thermoplastic polyurethane (TPU) film. The screen-printed Ag-TPU film was permanently attached to a fabric using heat lamination process. Three pressure sensors, PS1, PS2 and PS3 were assembled by sandwiching the porous dielectric layers D1, D2 and D3 between the top and bottom electrodes, respectively. An average pore size of $411 \mu \mathrm{m}, 496 \mu \mathrm{m}$, and $502 \mu \mathrm{m}$ was measured for D1, D2 and D3, respectively. A relative capacitance change of $\sim 100$%, $\sim$ 323%, and $\sim$ 485% was obtained for the pressure sensors PS1, PS2, PS3, respectively, for varying applied pressures ranging from 0 to 1000 kPa. The effect of curing temperatures on the thickness as well as the dielectric constant of the porous PDMS layer, which in turn changes the sensitivity of the pressure sensors, was investigated and is presented in this paper.